A water treatment for cleaning fluid by removing pollutants therefrom may use a heating method, a phase-changing method, or a separation membrane employing method.
The separation membrane employing method can realize high reliability since it is suitable for stably providing a desired water quality based on a size of pore formed in the separation membrane. Furthermore, it is unnecessary for the separation membrane employing method to perform a heating process. In this respect, the separation membrane employing method is advantageous in that it can be widely utilized in various separation processes using microbe that may be affected by the heating process.
The separation membrane may include a flat-type membrane having a flat cross section, and a hollow fiber membrane having a hollow therein. In case of the hollow fiber membrane, there are micro-pores provided on a surface of tubular fiber structure including inner and outer diameters therein, so that pollutants are filtered through the micro-pores included in the hollow fiber membrane. In comparison to the flat-type membrane, the hollow fiber membrane has the larger surface area owing to its inner and outer diameters. Accordingly, owing to the advantageous characteristics such as the large surface area of hollow fiber membrane, the hollow fiber membrane is widely utilized for the separation membrane in the recent applications of water treatment.
In the area of water treatment, the membrane is required to have, in addition to the high water permeability, an excellent mechanical strength, which is essential to build a reliable separation membrane system.
A hollow fiber-shaped membrane may be suitable for use in water treatment as it has a high permeability per installation area. However, due to its porous structure the mechanical strength needs to be improved. Thus, there have been attempts to reinforce the hollow fiber membrane by using a fabric or tubular braid as a support of the membrane.
Some examples of such reinforced hollow fiber membranes are disclosed in, for example U.S. Pat. No. 4,061,821, and U.S. Pat. No. 5,472,607, which propose a composite separation membrane reinforced with a fabric or tubular braid having the excellent mechanical strength.
U.S. Pat. No. 4,061,821 to Hayano et al. discloses the general idea of a composite hollow fiber membrane using a tubular braid. In U.S. Pat. No. 4,061,821, however, the tubular braids are not used as a support, instead they are completely embedded in the membrane in order to compensate for reduction of water permeability due to shrinkage occurred when the hollow fiber membrane, which is formed of an acryl polymer, alone is used at a temperature higher than 80° C. Such a composite membrane has disadvantages of significantly reduced water permeability due to the increased thickness of the membrane (the thickness of the membrane is larger than the membrane with a coating of a tubular braid) and due to the increased resistance of fluid flow through the membrane.
U.S. Pat. No. 5,472,607 reports reinforcing the composite hollow membrane by coating a reinforcing material on the surface of the membrane. In this case, the reinforcing material is not embedded in the composite membrane, but it is coated with the thin film, whereby filtration reliability is deteriorated due to a defect region in the thin film.
FIG. 1 is an expanded cross-sectional view of illustrating the composite hollow fiber membrane disclosed in U.S. Pat. No. 4,061,821, and FIG. 2 is an expanded cross-sectional view of illustrating the composite hollow fiber membrane disclosed in U.S. Pat. No. 5,472,607.
As shown in FIGS. 1 and 2, these membranes have a finger-like structure, and have voids D in the outer portion of the membrane or in the thin film layer, wherein the defect region (D) is comprised of a micro-pore having a diameter of 5 μm or above.
Voids may function as a defect when the polymer film exhibits the mechanical strength to support the membrane. Particularly, when the skin layer, which is the densest outermost layer of the multi-layer thin film coating, is damaged, the filtration reliability of the membrane may be reduced.
The composite hollow fiber membrane used as the separation membrane additionally requires a high peeling strength in addition to the excellent permeability, great mechanical strength, and high filtration reliability. That is, the composite hollow fiber membrane is required to have a good mechanical strength to be suitable for a submerged separation module in the fields of water treatment, in order to endure the friction and physical impact generated between membranes due to aeration in the water treatment system. In this respect, it is necessary for the composite hollow fiber membrane to obtain the high peeling strength and filtration reliability.
U.S. Pat. No. 6,354,444 proposes a composite hollow fiber membrane coated with a polymer resinous thin film on a braid made of monofilaments having a fineness of 0.5-7 denier.
In order to apply the hollow fiber membrane in various fields of water treatment, there are requirements for the high water permeability, the excellent mechanical strength, the high filtration reliability, the high peeling strength, the low dope permeation, and the high stiffness. The existing composite hollow fiber membrane can satisfy some of the aforementioned requirements. Thus, there is an increasing demand for the development of composite hollow fiber membrane which can satisfy all the aforementioned requirements.